System and method for scheduling burst profile changes based on minislot count

ABSTRACT

A system and method are presented for changing physical layer (PHY) parameters in a PHY device of a communications system. New parameters are written to a first-in first-out queue in a serial interface, while the scheduled time for the changeover is written to a control register in the serial interface. When the time for the changeover occurs, the parameters are written to the PHY device via a port of the serial interface.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No.60/261,273, filed Jan. 12, 2001, incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention described herein pertains to communications systems, andmore particularly to physical layer parameter changes.

2. Background Art

In modem digital communications systems, communicating entities need tohave a common, predetermined set of protocols and parameters. Giventhese protocols and parameters, the entities can communicate in anorderly, efficient manner. Such protocols and parameters are typicallyimplemented according to general functionality. The various functionsare often collectively modeled as multiple layers of a protocol stack.Each layer represents additional protocols that a communicating entitymust process, and/or parameters that must be adopted. The lowest layerin the protocol stack is typically the physical layer. The physicallayer establishes fundamental parameters relating to the format ofsignals over a physical medium. These parameters can include, forexample, the modulation method to be used, the error detection andcorrection method, the number of symbols to be transmitted per second,the number of bits that are represented by each symbol, and, ifbandwidth is allocated in terms of time slots, the slot size. In thecontext of a burst communications system, such parameters collectivelyrepresent a burst profile.

One example of a communications system standard that specifies aphysical layer is the Data Over Cable System Interface Specification(DOCSIS). DOCSIS was originally conceived for cable communicationssystems. While DOCSIS can be applied to such communications systems, itis not necessarily limited to cable. Wireless communications systems,for example, can also operate under DOCSIS. Likewise, DOCSIS can be usedin satellite communications systems.

DOCSIS can be used in communication systems that include a set of remotecommunications devices connected to a headend device, such that theheadend is responsible for the management of communications both to andfrom the remotes. The headend is responsible for the distribution ofinformation content to the remotes (the so-called “downstream”direction); in addition, the headend is responsible for management ofcommunications in the other direction, from the remotes to the headend(the “upstream” direction). Generally, in addition to sending content toremotes, the headend issues downstream messages that instruct eachremote as to when it can transmit upstream, and what kind of informationit can send. In effect, the upstream bandwidth is controlled andallocated by the headend. Any given remote can transmit upstream onlyafter requesting bandwidth and receiving a grant of the bandwidth fromthe headend. In a time division multiple access (TDMA) environment,bandwidth corresponds to one or more intervals of time. Moreover, theupstream can be organized into a number of channels, with severalremotes assigned to each channel. This arrangement allows the headend tomanage each upstream communications channel. In this manner, upstreamcommunications are managed so as to maintain order and efficiency and,consequently, an adequate level of service.

In the realm of cable communications, DOCSIS specifies the requirementsfor interactions between a cable headend and associated remote cablemodems. A cable headend is also known as a cable modem terminationsystem (CMTS). DOCSIS consists of a group of specifications that coveroperations support systems, management, and data interfaces, as well asnetwork layer, data link layer, and physical layer transport. Note thatDOCSIS does not specify an application layer. The DOCSIS specificationincludes extensive media access layer and physical (PHY) layer upstreamparameter control for robustness and adaptability. DOCSIS also provideslink layer security with authentication. This prevents theft of serviceand some assurance of traffic integrity.

The current version of DOCSIS (DOCSIS 1.1) uses a request/grantmechanism for allowing remote devices (such as cable modems) to accessupstream bandwidth. DOCSIS 1.1 also allows the provision of differentservices to different parties who may be tied to a single modem. Withrespect to the processing of packets, DOCSIS 1.1 allows segmentation oflarge packets, which simplifies bandwidth allocation. DOCSIS 1.1 alsoallows for the combining of multiple small packets to increasethroughput as necessary. Security features are present through thespecification of 56-bit Data Encryption Standard (DES) encryption anddecryption, to secure the privacy of a connection. DES is also used forauthentication. DOCSIS 1.1 also provides for payload header suppression,whereby repetitive ethernet/IP header information can be suppressed forimproved bandwidth utilization. DOCSIS 1.1 also supports dynamic channelchange. Either or both of the downstream and upstream channels can bechanged on the fly. This allows for load balancing of channels, whichcan improve robustness.

Sometimes it may be necessary to change the PHY parameters in acommunications system. For example, user requirements may change suchthat a different symbol rate is needed. PHY parameters may also have tobe changed as a result of changes in the communications environment. Forexample, if the communications environment becomes noisy, a differentmethod of error correction coding may be required.

DOCSIS provides a method in which PHY parameters (i.e, a burst profile)can be changed. Such a change requires a reprogramming of componentsthat handle PHY processing, including PHY devices at the headend. Theparameter change process for headend PHY devices is illustratedgenerally in FIG. 1. The process as illustrated pertains to changing PHYparameters for upstream communications. The process starts with step105. In step 110, the new PHY parameters for a given upstream channelare determined. In step 115, an upstream channel descriptor (UCD) isformulated. The UCD is a message sent from the headend to remote devicesand contains the new PHY parameter values. In step 120, the UCD is sentdownstream. In step 125, a determination is made as to the point in theupstream at which the new parameters are to take effect. In step 130, adownstream MAP message is formulated stating when, in the upstream, thechange is to occur. Note that such a message is commonly denoted incapitalized form, “MAP”; this convention is used hereinafter. The roleof MAP messages, generally, is to manage the upstream transmissions ofremote devices. Such a message typically allocates, i.e., maps, specifictime intervals in the upstream to specific remote devices, therebyallowing a given remote device to transmit upstream only in a specifiedtime interval.

Note that upstream time intervals are defined based on a clock having apredetermined frequency, such as 10.24 MHz. Such a clock can, in somesystems, be interpreted in terms of time units, or “ticks.” Each tickcan, for example, be 6.25 microseconds. Ticks can be further organizedinto larger units called minislots. The number of ticks per minislot canbe defined at the discretion of the headend. The available upstreambandwidth can therefore be viewed as a series of minislots. Moreover,MAP messages allocate the upstream bandwidth in terms of minislots.

In the case of changing PHY parameters in DOCSIS, a specific timeinterval (i.e., minislot sequence) is identified in which all remotesare barred from transmitting upstream. This is the interval in whichreprogramming of the PHY devices with the new parameters is to takeplace. Because no remote devices are allowed to transmit during thisinterval, the interval is referred to as “dead time.” DOCSIS specifiesthat the dead time last one millisecond.

Returning to FIG. 1, in step 135, the MAP message is sent downstream. Instep 140, the changeover point arrives (i.e., the start of firstminislot of the dead time, as specified in the MAP message) and acentral processing unit (CPU) at the headend is interrupted. Thisinterrupt must be handled during the dead time. In step 145, the newparameters are written, via a port of the CPU, to the headend PHYdevice. The write process is driven by software executing on the CPU.The process concludes at step 150.

The method of FIG. 1 however places a significant burden on the softwareexecuting on the CPU of the headend. Within the dead time interval, thesoftware receives the interrupt, must process the interrupt immediatelyand write the new parameters to the local PHY devices. Typically, thewrite process is performed by the CPU via a relatively slow serialinterface. The write process can take up to six hundred microseconds.Therefore, to complete processing within a one millisecond dead timeinterval can be a challenge. Moreover, if the dead time is exceeded,remote devices may begin transmitting, using the new PHY parameters,before the headend is ready. As a result, upstream data may be lost.Hence there is a need for a system and method that allows efficientreprogramming of PHY devices at the headend, such that there is minimalrisk of exceeding the dead time.

BRIEF SUMMARY OF THE INVENTION

The present invention is a system and method for changing physical layer(PHY) parameters in a PHY device of a communications system. Newparameters are written to a first-in first-out queue in a serialinterface, such as a serial peripheral interface (SPI), while thescheduled time for the changeover is written to a control register inthe serial interface. In an embodiment of the invention, the serialinterface is located at a media access controller at the headend. Whenthe time for the changeover occurs, the parameters are written to thePHY device via a serial interface port.

This avoids an otherwise significant burden on the software executing onthe headend CPU. Without the invention, the software would have toreceive and process an interrupt, then write any new parameters to thelocal PHY devices. In addition, this write process may have to beperformed via a relatively slow serial interface. The interrupt handlingand write process must take place within a brief (e.g., one millisecond)dead time interval. Moreover, if the dead time is exceeded, remotedevices may begin transmitting before the headend is ready, resulting ina loss of upstream data.

In contrast, the invention described herein has the feature ofprestoring new physical layer parameters in advance of actualreprogramming of the PHY device. The invention has the additionalfeature of prestoring the time of changeover. As a result, the inventionhas the advantage of allowing rapid reprogramming of the PHY device,without real-time intervention of the CPU, once the time for changeoverarrives.

The foregoing and other features and advantages of the invention will beapparent from the following, more particular description of a preferredembodiment of the invention, as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a flowchart illustrating the process of reprogramming headendPHY devices with new parameters according to the DOCSIS standard.

FIG. 2 is a block diagram illustrating headend functional components,including PHY devices, and associated remote devices.

FIG. 3 is a block diagram illustrating the system of the presentinvention, according to one embodiment.

FIG. 4 is a flowchart illustrating the processing of the invention,according to an embodiment.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the present invention is now described withreference to the figures, where like reference numbers indicateidentical or functionally similar elements. Also in the figures, theleft-most digit of each reference number corresponds to the figure inwhich the reference number is first used. While specific configurationsand arrangements are discussed, it should be understood that this isdone for illustrative purposes only. A person skilled in the relevantart will recognize that other configurations and arrangements can beused without departing from the spirit and scope of the invention. Itwill be apparent to a person skilled in the relevant art that thisinvention can also be employed in a variety of other devices andapplications.

I. Overview

The present invention is a system and method for changing upstream PHYparameters in a headend PHY device of a communications system. In thecontext of a burst communications system, new PHY parameters represent anew burst profile. Once the new parameters are known, they are writtento a first-in first-out (FIFO) transmit queue in a serial interface,such as a serial peripheral interface (SPI). In an embodiment of theinvention, the serial interface is located in a media access controller(MAC) at the headend. Once the scheduled time for the changeover isdetermined, the time is written to a control register in the interface.When the time for the changeover arrives, the parameters are written tothe local PHY device via an interface port. This serves to reprogram thePHY device. The invention allows new parameters to be prestored inadvance of reprogramming; moreover, the reprogramming effectivelybecomes a scheduled event. At the designated time, the reprogramming istriggered. Interrupt handling by a headend CPU is no longer necessary,and a time-consuming write by the CPU to the PHY device is avoided.

II. System

The system of the invention includes a serial interface, such as aserial peripheral interface (SPI), in a media access controller at theheadend. While the system of the invention, as described hereinafter,includes an SPI, note that this does not represent a limitation of theinvention. A person of ordinary skill in the art will recognize that theinvention can operate with any of a variety of serial interfaces thatinclude, but are not limited to, an SPI. An interIC (I²C) serialinterface, for example, can also be used.

The interface contains storage for the prestoring of new PHY parameters.The serial interface also contains one or more control registers thatcan be programmed with the time at which parameter changeover is tooccur. The storage of this information in the interface allows for fastand efficient reprogramming of PHY devices at the headend.

The system context of the invention is illustrated in FIG. 2. A centralprocessing unit (CPU) 205 is associated with the headend. CPU 205communicates with a media access controller (MAC) 215, typically via bus210. In an embodiment of the invention, bus 210 is a peripheralcomponent interconnect (PCI) bus. Media access controller 215 isresponsible for processing of the protocol and format requirementsassociated with the media access layer of the communications protocol.As will be described in greater detail below, it is the media accesscontroller 215 that houses the serial interface. Media access controller215 is connected to one or more PHY devices, shown here as devices 220and 225. These devices perform processing required for the physicallayer. One example of a PHY device is the BCM 3138, available fromBROADCOM Corporation, of Irvine, Calif. Components 205 through 225 areall located at the headend. In the context of a cable communicationssystem, these components are part of the CMTS. Remote devices associatedwith the headend are illustrated as devices 230 a-230 n. In the contextof a cable communications system, remote devices 230 a-230 n can becable modems, such as any of the BCM 93350 family of cable modems, alsoavailable from BROADCOM Corporation.

Media access controller 215 and PHY device 220 are collectively labeledas assembly 250. Assembly 250 is illustrated in greater detail in FIG.3. Media access controller 215 communicates with CPU 205 via an internalinterface 305. In an embodiment of the invention, interface 305 can be aPCI interface. In an alternative embodiment, interface 305 can be ofanother type, such as the INTEL i960 or comparable interface. Theinformation that media access controller 215 receives from CPU 205includes any new PHY parameters that need to be programmed intoassociated local PHY devices, such as PHY device 220. PHY parameters caninclude the number of symbols to be transmitted upstream per second, thenumber of bits per symbol, and specification of the modulation processand the error detection/correction method. Media access controller 215also receives information from CPU 205 regarding when a PHY device is tostart using the new parameters. The new parameters and the timinginformation for the changeover are saved in a serial interface, such asSPI 310.

In particular, new PHY parameters are stored in transmit queue 315. Inan embodiment of the invention, transmit queue 315 is a first-infirst-out (FIFO) queue. Timing information for the changeover ofparameters is written to one or more control registers 320. In thecontext of a communications system that operates under the DOCSISstandard, the changeover time information can be expressed and recordedin control registers 320 by naming an interval of minislots during whichthe changeover is to take place. Accordingly, control registers 320 alsoreceive time information 325 which represents a regular update as to thecurrent point in time of the upstream. In the context of a DOCSISsystem, time information 325 represents the current minislot count. Whenthe current time information 325 matches the changeover timinginformation received from CPU 205, control registers 320 enable transmitqueue315 to send the new PHY parameters. The new PHY parameters are thensent to PHY device 220 via SPI port 330. Note that SPI 310 also includesa receive queue 335. Receive queue 335 receives and stores readinformation, and is shown here for the sake of completeness.

In an embodiment of the invention, an analogous system can be used at aremote device (e.g., a cable modem). Such a system can prestore new PHYparameters for subsequent reprogramming of a remote PHY device at apredetermined time.

III. Method

The method of the invention is illustrated in FIG. 4. The method beginswith step 405. In step 410, any new PHY parameters for a given upstreamchannel are determined. In step 415, an upstream channel descriptor(UCD) is formulated. The UCD is a message generated at the headend. Itis sent ultimately to PHY devices associated with the remote devices.This transmission takes place in step 420. In step 425, a determinationis made as to when the new PHY parameters are to be first used. In step430, a MAP message is formulated that states when the changeover is totake place (the dead time) and when the new PHY parameters are to befirst used. In a communications system using the DOCSIS standard, thechangeover point is defined in terms of a minislot count. The MAPmessage in DOCSIS system therefore specifies the minislots during whichthe changeover is to take place. It is during this time that the remotedevices are to reprogram their PHY devices with the new PHY parameters.During this designated dead time, no remote devices are to transmitupstream. The MAP message is sent to remote devices in step 435.

In step 440, the new PHY parameters are queued into a transmit queue ofa serial interface, such as a SPI. As described above, in an embodimentof the invention, this queue is structured as an FIFO queue. In step445, the changeover point is written to a control register of the serialinterface, e.g., a SPI control register. As discussed above, thechangeover point can be defined in terms of a minislot count in a DOCSISsystem. In addition, in an embodiment of the invention, a specificupstream channel to which the new parameters are to apply can also bewritten to the control register. In step 450, the changeover pointoccurs, and the new PHY parameters that had been stored in the transmitqueue are written to the appropriate headend PHY device through a portof the serial interface, e.g., the SPI port, in the case of an SPI. Theprocess concludes at step 455.

In an embodiment of the invention, an analogous method can be used at aremote device (e.g., a cable modem). In such a method, new PHYparameters can be prestored for subsequent reprogramming of a remote PHYdevice at a predetermined time.

IV. Conclusion

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample, and not limitation. It will be apparent to persons skilled inthe relevant art that various changes in detail can be made thereinwithout departing from the spirit and scope of the invention. Thus thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A system for changing one or more physical layer communicationsparameters as configured in a physical layer device, the systemcomprising: a first interface for receiving the parameters; and a serialinterface that stores the parameters in advance of sending theparameters to the physical layer device, stores a predeterminedchangeover time; and sends the parameters to the physical layer deviceat the predetermined changeover time.
 2. The system of claim 1, whereinsaid first interface is a peripheral component interconnect (PCI)interface.
 3. The system of claim 1, wherein said serial interface is aserial peripheral interface (SPI).
 4. The system of claim 1, furthercomprising: a central processing unit (CPU) that sends the parameters tosaid first interface.
 5. The system of claim 1, wherein said serialinterface comprises: a transmit first-in first-out (FIFO) queue in whichthe parameters are stored; a serial interface port through which theparameters are sent to the physical layer device; and one or morecontrol registers for storing said predetermined changeover time.
 6. Thesystem of claim 5, wherein said control registers further store achannel identifier, which corresponds to a communications channel towhich the parameters pertain.
 7. The system of claim 1, wherein thephysical layer communications parameters correspond to a burst profile.8. A method of changing one or more physical layer communicationsparameters in a physical layer device, comprising the steps of: (a)receiving the parameters; (b) storing the parameters in a serialinterface; (c) receiving a point in time at which the parameters are tochangeover; (d) storing the changeover point in the serial interface;and (e) at the changeover point, writing the parameters to the physicallayer device.
 9. The method of claim 8, wherein the parameterscorrespond to a burst profile.
 10. The method of claim 8, wherein saidstep (b) comprises the step of storing the parameters in a transmitfirst-in first-out (FIFO) queue in the serial interface.
 11. The methodof claim 8, wherein the changeover point is expressed in terms of aminislot count.
 12. The method of claim 8, wherein said step (d)comprises storing an indication of the changeover point in one or morecontrol registers of the serial interface.
 13. The method of claim 8,wherein said step (e) comprises writing the parameters to the physicallayer device via a serial interface port in the serial interface. 14.The method of claim 8, further comprising the step of: (f) receivingperiodic updates of the current time, performed before step(e).
 15. Themethod of claim 14, wherein the periodic updates are minislot counts.